Al-Mg-Si alloy with good extrusion properties

ABSTRACT

An alloy of composition in wt. % (see table (I)) and incidental impurities up to 0.05 each 0.15 total, balance A1. The alloy can be extruded at high speed to provide extruded sections which meet T5 or T6 strength requirements.

This invention concerns AlMgSi alloys of the 6000 series of the AluminumAssociation Register. The compositions are low magnesium containingAlMgSi alloys with appropriate silicon and copper additions to meet thestrength requirements of AA6063T5 and T6. AA6063 accounts forapproximately 80% of all aluminium extruded products. At this bottom endof the extrusion market, there is a need for extrusions which meet theT5 or T6 strength requirements but which can be manufactured at thehighest possible rates of extrusion.

This need was addressed in a paper by D Marchive in Light Metal Age,April 1983, pages 6-10. The author reported a trend towards reducing thecontent of Mg₂Si and compensating for this by increasing the excess ofSi, but that resulted in loss of formability. He reported new alloys inwhich the concentrations of Mg, Si, Cu, Mn and Cr were optimised toprovide alloys which exhibited the required tensile properties but withsuperior extrudability, formability and toughness. The alloys had Mgcontents in the range 0.35 to 0.60.

There has been a prejudice in the industry against reducing the Mgcontent of 6000 series general purpose extrusion alloys below 0.35 wt %.Of the 63 6000 series alloys listed in the Aluminum AssociationRegister, all the general purpose extrusion alloys require a Mg contentof at least 0.35 wt %.

WO 95/06759 describes high strength high extrudability AlMgSi Alloyshaving the composition in wt %: Mg 0.25-0.40; Si 0.60-0.90; Fe up to0.35; Mn up to 0.35 preferably 0.10-0.25. But these are not generalpurpose extrusion alloys. By virtue of the high Si content they havehigh tensile strength generally in excess of 250 MPa and they preferablycontain Mn to improve extrusion surface quality.

This invention concerns general purpose extrusion alloys having theminimum alloying additions required to meet the strength requirements ofAA 6063T5 (peak aged tensile strength of at least about 152 MPa) and T6(peak aged tensile strength of at least about 207 MPa). Decreasing theMg content of such an alloy reduces the flow stress of the material atthe temperatures used for extrusion, which in turn reduces the extrusionpressure and the work done in the process. Approximately 90% of the workof extrusion is converted to heat which results in temperature rise inthe extruded product. With the dilute alloys described here, less heatis generated in the extrusion process as compared with conventionalalloys, such that the product can be extruded at a higher speed beforesurface deterioration occurs. Usually the productivity of an aluminiumextrusion is limited by the onset of various types of surface defectwhich is linked to the attainment of a critical temperature at thesurface of the product.

The lower breakthrough pressure associated with the lower Mg contentalso means that for a given extrusion press, the initial billettemperature can be reduced until the pressure required matches the presscapacity. This has the effect of further reducing the temperature of theproduct as it exits the die which gives further productivity benefits.

In one aspect the invention provides an alloy of composition in wt %

Broad Narrow Mg 0.20-0.34 0.20-0.30 Si 0.35-0.60 0.40-0.59 Mn 0.15 max0.03-0.10 Cu 0.25 max 0.20 max Fe 0.35 max 0.25 max

incidental impurities up to 0.05 each 0.15 total balance Al providedthat when Mg is at least 0.30 and Cu is at least 0.05, then Fe isgreater than 0.15.

The invention also provides extrusion ingots of the alloy as defined;and extrusions (i.e. extruded sections) made from such ingots.

Reference is directed to the accompanying drawings in which:

FIG. 1 is a graph showing Mg and Si concentrations of certain dilute6000 alloys.

FIG. 2 is a graph of tensile strength against Si content for alloys ofdifferent composition.

FIG. 3 is a graph showing T5 and T6 limits for various dilute 6000alloys.

FIG. 4 is a graph showing elongation at break for alloys of differentcomposition.

FIG. 5 is a bar chart showing relative mean extrusion breakthroughpressure for alloys of different composition.

FIG. 6 is a graph showing extrusion breakthrough load at 450° C. foralloys of different composition.

FIG. 7 is a graph showing extrusion breakthrough load at 425° C. foralloys of different composition.

FIGS. 8 and 9 are graphs of tensile strength against Si content of twodifferent alloys showing the effects of different ageing practices.

FIG. 10 is a graph showing the effect of silicon content on α-βtransformation.

FIG. 11 is a graph showing the effect of silicon content on AlFeSiintermetallic size.

FIG. 12 is a graph showing the effect of silicon content on AlFeSispheroidisation.

FIG. 13 is a graph showing the effect of Si and Mn levels on α-βtransformation.

FIG. 14 is a graph showing the effect of Mg and Si content on roughnessof extrusions according to the invention at an extrusion temperature of450° C.

FIG. 15 is a graph showing the effect of alloy composition on tensilestrength.

Referring to FIG. 1, the boxes marked A and B designate the broad andnarrow compositions of alloys according to the invention as definedabove. Also shown for comparison is the bottom end of a box for AA6060,a general purpose extrusion alloy; and the left hand side of a box forthe high strength alloy described in WO 95/06759. Lines marked T5 and T6represent the compositions required to produce extrusions capable ofpassing these tests. The positions of these lines are somewhat variabledepending on ingot pretreatment conditions, rate of cooling theextrusions and extrusion ageing conditions.

The Mg content of the invention alloy is set at 0.20-0.34 preferably0.20-0.30%. If the Mg content is too low, it is difficult to achieve therequired strength in the aged extrusions. Extrusion pressure increaseswith Mg content, and becomes unacceptable at high Mg contents.

The Si content is set at 0.35-0.60 preferably 0.40-0.59. If the Sicontent is too low, the alloy strength is adversely affected, while ifthe Si content is too high, extrudability may be reduced. Formabilityhas also been reported to be impaired at high Si levels, but it has beenfound that this effect is not important within the composition range ofthe invention. A function of the Si is to strengthen the alloy withoutadversely affecting extrudability, high temperature flow stress oranodising and corrosion characteristics.

The presence of Fe in the alloy is normally unavoidable. An upperconcentration limit is set at 0.35, preferably 0.25%. It is likely to bepreferred to use alloys containing at least 0.15% Fe, to prevent brightfinish on anodising and because these alloys are less expensive thanalloys containing lower Fe concentrations especially when made fromremelted scrap. In the as-cast alloy ingots, Fe is present in the formof large plate-like β-AlFeSi particles. Preferably the extrusion ingotis homogenised to convert β-AIFeSi to substantially (at least 80% andpreferably more than 90%) the α-AlFeSi form.

Mn has a number of different effects. Although it has previously beenincluded in extrusion alloys to improve toughness, it is generally notuseful for this purpose for alloys of the present invention. At veryhigh levels, Mn gives rise to problems with quench sensitivity due toincreased levels of dispersoid formation. To avoid this, Mn levels arepreferably kept below 0.15% particularly below 0.10%.

The inventors have determined that, when included at a level of at least0.02% preferably at least 0.03%, Mn has a hitherto unpublished technicaleffect. This is that silicon levels of about 0.50 wt % or greater leadto increased stability of the P-AlFeSi phase at homogenisationtemperatures. This retards the transformation of the AlFeSiintermetallic from β to α during homogenisation. As a result, the breakup of the intermetallics is retarded such that mean size of theintermetallic phases is increased and a degree of spheroidisation isreduced. This has detrimental effects on the extrudability of thematerial and causes poor surface finish. The effects of the siliconlevel on a stability can be avoided by adding an appropriate level ofmanganese to the alloy which stabilises the α form of the Al(Fe,Mn)Siintermetallic. Thus a preferred minimum manganese content can beexpressed:

Wt % manganese=at least 0.3×wt % silicon−0.12

Inclusion of manganese in these concentrations helps to: promote a β toα AlFeSi transformation during homogenisation such that at least an 80%and preferably at least 90% transformation is achieved under normalhomogenising conditions; reduce the AlFeSi particle size (which ishowever also dependent on the diameter of the billet being homogenised);and increase the degree of AlFeSi spheroidisation, preferably to atleast 0.5 or 0.51 (where 0 equals a rod and 1 equals a sphere).

Cu has the advantage of improving tensile strength without a comparableincrease in extrusion breakthrough pressure; and a disadvantage ofgiving rise to corrosion problems. Particularly at low Si levels, Cu maybe included in alloys of this invention at concentrations up to 0.25%preferably up to 0.20%, and particularly up to 0.10%.

The strength of extrusion alloys is sometimes expressed in terms oftheir Mg₂Si content, which for excess Si alloys such as these may becalculated as Mg×1.57 . The Mg₂Si content of alloys according to thisinvention is preferably 0.314-0.55 wt %, particularly 0.38-0.53 wt %.

Si is present in an excess over that required to combine with all Mg asMg₂Si, and with all Fe and Mn as AI(Fe,Mn)Si. (The terms AlFeSi andAl(Fe,Mn)Si are conventionally used to denote intermetallics containingthese elements but not necessarily in these proportions). Excess Si iscalculated according to the following formula

Excess Si=Si−Mg/1.73−(Fe+Mn)/3.

In alloys of the present invention, the excess Si is preferably0.08-0.48 wt % particularly 0.12-0.40 wt %. Where this excess is toosmall, it will be difficult to achieve the required tensile strengthproperties. Where this excess is too large, alloy formability andextrusion surface quality may be adversely affected.

An extrusion ingot of the alloy of the invention may be made by anyconvenient casting technique, e.g. by a d.c. casting process, preferablyby means of a short mould or hot-top d.c. process. The Fe is preferablypresent as an insoluble secondary phase in the form of fine β-AlFeSiplatelets, preferably not more than 15 μm in length or, if in the αform, free from script and coarse eutectic particles.

The as-cast extrusion ingot is homogenised, partly to bring the solublesecondary magnesium-silicon phases into suitable form, to dissolve thesilicon and partly to convert β-AlFeSi particles into substantiallyα-AIFeSi particles, preferably below 15 μm long and with 90% below 6 μmlong. Homogenisation typically involves heating the ingot at more than530° C. e.g. 550-600° C. for 30 minutes to 24 hours with highertemperatures requiring shorter hold times.

Cooling from homogenisation temperature should preferably besufficiently fast to avoid the formation of coarse β-Mg₂ Si particleswhich would not redissolve during extrusion. It is preferred to cool theingot at a rate of at least 150° C. per hour from homogenisationtemperature down to a temperature not greater than 425° C. The ingot maybe held for a few hours at a temperature in the range 300-425° C., asdescribed in EP 222 479, in order to encourage the formation of a ratherfine precipitate β′-Mg₂Si which has the effect of reducing extrusionbreakthrough pressure and of redissolving during extrusion so as topermit development of maximum tensile strength in age hardenedextrusions. The rate of cooling below 300° C. is immaterial.

The homogenised extrusion ingot is then heated for extrusion. Thesolutionising treatment described in EP 302 623 may be used. As isconventional in the art, the initial billet temperature can be chosen tomatch the pressure capacity of the extrusion press being used. Theemerging extrusion is cooled, either by water or forced air or morepreferably in still air, and subjected to an ageing process in order todevelop desired strength and toughness properties.

Ageing typically involves heating the extrusion to an elevatedtemperature in the range 150-200° C., and holding at that temperaturefor 1-48 hours, with higher temperatures requiring shorter hold times.As demonstrated in the experimental section below, the response of theextrusion to this ageing process depends significantly on the rate ofheating. A preferred rate of heating is from 10-100° C. particularly10-70° C. per hour; if the heating rate is too slow, low throughputresults in increased costs; if the heating rate is too high, themechanical properties developed are less than optimum. An effectequivalent to slow heating can be achieved by a two-stage heatingschedule, with a hold temperature typically in the range of 80-140° C.,for a time sufficient to give an overall heating rate within the aboverange. Holding the extrusion for 24 hours or more at room temperature isalso beneficial.

When aged to peak strength, extrusions are capable of meeting therequirements of T5 (tensile strength of 152 MPa) or preferably of T6(tensile strength of about 207 MPa) with improved press productivity.The reduced flow stress characteristics also make it possible to produceshapes such as high aspect ratio heat sinks that are difficult toproduce in existing alloys. The basic features of the alloys can also beapplied for bright dip applications, with appropriate additions ofcopper or for matt etching applications with appropriate control of theiron content. Some of the more dilute versions of the alloys aresuitable for applications where low strength is acceptable but wheregood formability is required.

EXPERIMENTAL

The invention has been tested in the laboratory. A range of compositionslisted in Tables 1 and 2 were DC cast as 100 mm diameter ingots. Thesecovered the following ranges of composition:

Mg  0.23-0.48 wt % Si  0.39-0.61 wt % Cu 0.001-0.10 wt % Fe  0.17-0.19wt % Mn 0.028-0.03 wt %

The range of alloys included a control alloy based on a commerciallyavailable alloy 6060 (Example 11) and 6063.

The billets were homogenised using a practice of 2 hrs at 585° C.followed by cooling at 350° C./hr, which is a typical practice forAl-Mg-Si alloys. The alloys were then extruded using a 750 tonne, 100 mmdiameter extrusion press. Billets were induction heated using a numberof different practices and then extruded into a 50×3 mm flat strip,equivalent to an extrusion ratio of 52:1. The extrusion speeds usedranged from 12 to 40 m/min. Initially the billet was extruded using abillet temperature of 480° C. at an exit speed of 40 m/min giving anexit temperature of at least 510° C. The strip was still air cooled at2° C./sec. After 24 hours the alloys were aged using the followingpractices:

1. 100° C./hr heat up, soak for 5 hrs at 185° C.

2. 50° C./hr heat up, soak for 5 hrs at 185° C.

3. 20° C./hr heat up, soak for 5 hrs at 185° C.

4. 100° C./hr heat up, soak for 5 hrs at 120° C. followed by 100° C./hrheat up, soak for 5 hrs at 185° C.

FIG. 2 shows the tensile strengths obtained using the first heattreatment as a function of silicon and magnesium content. Therequirements of the AA6063T5 and T6 specifications are shown. Thefollowing points can be drawn from this diagram, bearing in mind thatthe accepted alloy specification for most extruded applications isAA6063:

The minimum tensile strength requirement of AA6063T6 can be satisfied atmagnesium levels of 0.29 and above with appropriate control of thesilicon level. The strength obtained from such alloys is equivalent tothe properties obtained with the alloy 6060.

The 6063T5 minimum tensile strength requirement can be easily met withall but the lowest Mg and Si compositions tested. This includes all thenew 0.25 wt % Mg alloys tested apart from the lowest silicon level.

The addition of 0.10 wt % copper to a 0.29 wt % Mg alloy resulted in a10 MPa increase in tensile strength. This indicates that it should bepossible to meet the 6063T6 requirement at 0.25 wt % Mg-0.6 wt % Si byadding a similar level of copper.

FIG. 1 showing alloy composition fields and strength contours have beenreproduced as FIG. 3, in which are shown the compositions and tensilestrengths of the various alloys listed in Table 1. From this graph it isevident that alloys containing as little as 0.20 wt % Mg can beformulated with suitably high Si contents to pass the T5 specification.

FIG. 4 is a graph of elongation at break for alloys of variouscompositions after ageing by practice 1. Elongation at break in atensile test is one measure of formability. The following conclusionscan be drawn from this figure:

Elongation did not decrease with increased excess Si and at the lowestMg level (0.25 wt %) elongation increased only slightly with increasingSi.

The AA6060 control gave similar elongation values to the experimentalalloys.

All the values were in excess of the minimum requirements, which are 8%,of AA6063T5 and T6.

The pressure requirements of the new alloy range have been compared withexisting alloys AA6060 and AA6063 in the temperature range 400 to 475°C. In this case the alloys were extruded into a thin wall profile (1.3mm thick I-section) at a reduction ration of 125:1. Individual billetswere extruded at 400, 425, 450, 475° C. The experiments were carried outon a laboratory press as described previously. The press liner, die andtooling were preheated to the billet temperature in each case. TheAA6063 composition is included in Table 1.

FIG. 5 summarises the results expressed as mean breakthrough pressureover the temperature range 400-475° C. The alloys are ranked on they-axis in terms of decreasing magnesium and silicon contents. There is aprogressive decrease in pressure as the magnesium and silicon contentsare reduced. All the alloys within the composition range covered by theinvention offer useful pressure reductions as compared to conventionalalloys AA 6060 and AA6063. As described above, these useful improvementsin extrudability can be achieved whilst still satisfying the mechanicalproperty requirements for to these types of applications. FIGS. 6 and 7give more detailed pressure data for a typical extrusion temperatures of450 and 425° C. The benefits of the new alloy range, in terms of reducedextrusion pressure, appear to increase as lower billet temperatures areutilised.

The addition of 0.10 wt % Cu to the 0.30 wt % Mg containing alloy doesraise the extrusion pressure such that it is equivalent to adding 0.05wt % Mg. From FIG. 2, it is also equivalent to an addition of 0.05 wt %Mg or 0.05 wt % Si in terms of mechanical properties and is still auseful way of controlling the mechanical properties.

The effect of ageing practice on the properties achievable is shown inFIGS. 8 and 9, (in which extrusion ingots were solutionised by thetechnique described in EP 302623A). Further increases in strength arepossible for all the compositions studied by reducing the heating rateto the ageing temperature to 20° C./hr (practice 3) or by using the twostage heat treatment (practice 4).

FIGS. 10 to 13 show the results of experiments performed by taking 178mm diameter ingots of alloys according to the invention containing0.25-0.50 wt % magnesium (the Mg content does not affect the results)and variable concentrations of silicon as shown; and subjecting theingots to homogenisation under conventional conditions, typically 2hours at 585° C. FIG. 10 shows the effect of increasing silicon levelson the percentage of β-AlFeSi remaining after homogenisation for ingotscontaining 0.03 wt % Mn. Above 0.5 wt % Si, the percent α-AIFeSi at theend of homogenisation is significantly reduced. FIGS. 11 and 12 show themean size and degree of spheroidisation for the same alloys. Theincrease in residual β-AlFeSi at higher Si levels corresponds to anincrease in particle size and lack of spheroidisation. In each of FIGS.10, 11 and 12, a single further point shows that, by changing the Mnconcentration from 0.03% to 0.09%, these detrimental effects onintermetalllic type, size and shape can be reversed.

FIG. 13 shows the effect of incremental Mn additions on the level ofβ-AlFeSi remaining after standard homogenisation practice. Lines areshown for two Si levels of 0.50% and 0.60%. A target often used forhomogenisation of dilute 6060 alloys is to achieve 90% α-AlFeSi afterhomogenisation. The amount of Mn required to achieve this increases withthe bulk Si content. For alloys containing less than 0.50 wt % Si, adeliberate addition of Mn is not necessary to achieve this target, butthe addition can still be useful in improving the extent ofspheroidisation for a given homogenisation treatment.

FIG. 14 is a graph showing the effect of Mg and Si content on roughnessRa of extrusions made from alloys shown in Table 1 at an extrusiontemperature of 450° C. The exit speed was 100 m/min, the extrusion ratiowas 125:1 and the section thickness was 1.3 mm. It can be seen thatsurface roughness begins to be a problem at Si levels above about 0.52wt %.

FIG. 15 is a chart showing the effect of alloy composition on tensilestrength. Four ingots of each of nine different alloy compositions 17 to25 as shown in Table 3 were extruded and aged and the UTS measured. Thebillet temperature was 450° C., the extrusion ratio was 125:1 and thequench rate was 3° C./s. Ageing was 5hr at 180° C. with a 100° C./hrheating rate. The results show that an alloy content of 0.31 wt % Mg and0.53 wt % Si is close to the lower limit for achieving 6063 T6properties. A comparison of Example alloys 18, 19 and 20 shows that Mnaddition does not have any significant effect on strength.

Table 4 below provides further data on the extrusion properties of thoseexample alloys 17 to 25: the load required to achieve extrusion at 100metres/minute; and the roughness of the resulted extruded section. Acomparison of example alloys 18, 19 and 20 shows that, at high Silevels, surface finish can be substantially improved by Mn addition.

A comparison of prior art alloys 21 and 25 with the others shows thatthe invention alloys require lower extrusion pressures.

TABLE 1 Example % % % % % % % % No Si Fe Cu Mn Mg Zn Ti B 1 .40 .18 .002.029 .35 .007 .008 .001 2 .44 .17 .001 0.030 .34 .007 .008 .001 3 .50.17 .001 .029 .34 .007 .008 .001 4 .54 .17 .002 .029 .34 .007 .008 .0015 .59 .17 .002 .029 .33 .006 .007 .001 6 .39 .17 .001 .029 .28 .006 .007.001 7 .43 .17 .001 .029 .28 .007 .007 .001 8 .50 .17 .002 .029 .29 .007.007 .001 9 .55 .17 .002 .029 .30 .007 .007 .001 10 .61 .17 .002 .029.30 .007 .008 .001 11 .45 .17 .001 .029 .39 .007 .007 .001 12 .55 .17.10 .029 .29 .007 .007 .001 13 .43 .19 .001 .028 .23 .008 .0010 14 .50.19 .001 .028 .24 .008 .0010 15 .56 .19 .002 .029 .24 .007 .0010 16 .61.19 .002 .029 .24 .008 .0010 6063 .41 .17 .03 .48 .01

TABLE 2 Mg₂Si Excess Si Example (wt %) (wt %) 1 .55 .13 2 .53 .17 3 .53.23 4 .53 .27 5 .52 .33 6 .44 .16 7 .44 .20 8 .46 .26 9 .47 .31 10 .47.37 11 .61 .15 12 .46 .31 13 .36 .23 14 .38 .29 15 .38 .35 16 .38 .406063 .75 .06

TABLE 3 % % % % % % % % Example Si Fe Cu Mn Mg Ti B Type 17 0.4 0.180.002 0.029 0.35 0.008 0.001 18 0.61 0.17 0.002 0.03 0.28 0.008 0.001 190.62 0.17 0.002 0.09 0.27 0.008 0.001 20 0.63 0.17 0.002 0.06 0.31 0.0080.001 21 0.45 0.17 0.002 0.03 0.41 0.007 0.001 6060 22 0.52 0.17 0.10.03 0.32 0.008 0.001 23 0.53 0.17 0.003 0.03 0.31 0.008 0.001 24 0.450.17 0.001 0.03 0.3 0.008 0.001 25 0.41 0.17 0.03 0.48 0.01 6063

TABLE 4 Example Roughness Extrusion Load Alloy Ra (μm) (tonnes) 17 — 181.10 507 19 0.44 507 20 0.69 510 21 0.81 522 22 1.08 511 23 1.12 508 240.80 509 25 — 532

What is claimed is:
 1. A DC cast extrusion ingot of an alloy ofcomposition consisting of 0.20-0.34 wt. % Mg, 0.35-0.60 wt. % Si,0.02-0.15 wt. % Mn, up to 0.10 wt. % Cu, up to 0.35 wt. % Fe, optionallyTi and B as grain refiners, incidental impurities up to 0.05 wt. % each,up to 0.15 wt. % total, balance Al, provided that when Mg is at least0.30 wt % and Cu is at least 0.05 wt %, then Fe is greater than 0.15 wt%.
 2. An extrusion ingot as claimed in claim 1, wherein the alloycomprises 0.20 to 0.30 Mg.
 3. An extrusion ingot as claimed in claim 1,wherein Mg₂Si is present and the Mg₂Si concentration is 0.35-0.55 wt %and an excess Si of 0.10-0.45 wt % is present.
 4. An extrusion ingot asclaimed in claim 1, wherein the wt % of Mn is at least (0.3×Si−0.12). 5.An extrusion ingot as claimed in claim 1, wherein the Fe content is atleast 0.15 wt %.
 6. An extrusion ingot as claimed in claim 1, in whichFe is present substantially as α-AlFeSi, by virtue of homogenisation. 7.An extrusion made from an ingot as claimed in claim
 6. 8. An extrusionas claimed in claim 7, which has in the T6 temper an ultimate tensilestrength of at least 207 MPa.
 9. An extrusion as claimed in claim 8,which has been thermally aged, wherein the rate of heating for ageing is10-100° C./hr.
 10. A method of making an extrusion, comprising DCcasting an extrusion ingot of an alloy of composition consisting of0.20-0.34 wt. % Mg, 0.35-0.60 wt % Si, 0.02-0.15 wt. % Mn, up to 0.10wt. % Cu, up to 0.35 wt. % Fe, optionally Ti and B as grain refiners,incidental impurities up to 0.05 wt. % each, up to 0.15 wt. % total,balance Al, provided that when Mg is at least 0.30 wt % and Cu is atleast 0.05 wt %, then Fe is greater than 0.15 wt %; homogenising theingot to convert β-AlFeSi particles to substantially an α-Al-Fe-Si form;cooling the homogenised ingot; and extruding the ingot.
 11. A method asclaimed in claim 10, wherein the alloy comprises 0.10 max Cu.
 12. Amethod as claimed in claim 10, wherein the alloy comprises 0.20 to 0.30Mg.
 13. A method as claimed in claim 10, wherein the homogenisation iseffected at 550° C.-600° C. for 30 minutes to 24 hours.
 14. A method asclaimed in claim 10, wherein the homogenisation ingot is cooled down to425° C. or less at a rate of at least 150° C. per hour.
 15. A method asclaimed in claim 10, wherein the extrusion is age hardened by heating at10-100° C./hr to an ageing temperature of 150-200° C.
 16. A DC castextrusion ingot of an alloy of composition consisting of 0.20-0.34 wt. %Mg, 0.40-0.59 wt. % Si, 0.03-0.10 wt. % Mn, up to 0.10 wt. % Cu, up to0.35 wt. % Fe, optionally Ti and B as grain refiners, incidentalimpurities up to 0.05 wt. % each, up to 0.15 wt. % total, balance Al,provided that when Mg is at least 0.30 wt % and Cu is at least 0.05 wt%, then Fe is greater than 0.15 wt %; in which AlFeSi phase is presentand more than 80% of said AlFeSi is present as α-AlFeSi, by virtue ofhomogenisation.
 17. An extrusion ingot as claimed in claim 16 whereinmore than 90% of said AlFeSi is present as α-AlFeSi, by virtue ofhomogenisation.
 18. An extrusion ingot as claimed in claim 17characterized in that an extrusion produced from the ingot, and aged topeak strength, has an ultimate tensile strength of at least 207 MPa. 19.An extrusion ingot as claimed in claim 16 characterized in that anextrusion produced from the ingot, and aged to peak strength, has anultimate tensile strength of at least 207 MPa.